/* Copyright (c) 2012 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.vending.billing.util;

import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

import org.apache.commons.lang3.StringUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Security-related methods. For a secure implementation, all of this code should be implemented on a server that
 * communicates with the application on the device. For the sake of simplicity and clarity of this example, this code is
 * included here and is executed on the device. If you must verify the purchases on the phone, you should obfuscate this
 * code to make it harder for an attacker to replace the code with stubs that treat all purchases as verified.
 */
public class Security {

	private static Logger logger = LoggerFactory.getLogger(Security.class);

	private static final String KEY_FACTORY_ALGORITHM = "RSA";
	private static final String SIGNATURE_ALGORITHM = "SHA1withRSA";

	/**
	 * Verifies that the data was signed with the given signature, and returns the verified purchase. The data is in
	 * JSON format and signed with a private key. The data also contains the {@link PurchaseState} and product ID of the
	 * purchase.
	 * 
	 * @param base64PublicKey
	 *            the base64-encoded public key to use for verifying.
	 * @param signedData
	 *            the signed JSON string (signed, not encrypted)
	 * @param signature
	 *            the signature for the data, signed with the private key
	 */
	public static boolean verifyPurchase(String base64PublicKey, String signedData, String signature) {
		if (signedData == null) {
			logger.error("data is null");
			return false;
		}

		boolean verified = false;
		if (!StringUtils.isEmpty(signature)) {
			PublicKey key = Security.generatePublicKey(base64PublicKey);
			verified = Security.verify(key, signedData, signature);
			if (!verified) {
				logger.warn("signature does not match data.");
				return false;
			}
		}
		return true;
	}

	/**
	 * Generates a PublicKey instance from a string containing the Base64-encoded public key.
	 * 
	 * @param encodedPublicKey
	 *            Base64-encoded public key
	 * @throws IllegalArgumentException
	 *             if encodedPublicKey is invalid
	 */
	public static PublicKey generatePublicKey(String encodedPublicKey) {
		try {
			byte[] decodedKey = Base64.decode(encodedPublicKey);
			KeyFactory keyFactory = KeyFactory.getInstance(KEY_FACTORY_ALGORITHM);
			return keyFactory.generatePublic(new X509EncodedKeySpec(decodedKey));
		} catch (NoSuchAlgorithmException | InvalidKeySpecException | Base64DecoderException e) {
			logger.error("", e);
			throw new IllegalArgumentException(e);
		}
	}

	/**
	 * Verifies that the signature from the server matches the computed signature on the data. Returns true if the data
	 * is correctly signed.
	 * 
	 * @param publicKey
	 *            public key associated with the developer account
	 * @param signedData
	 *            signed data from server
	 * @param signature
	 *            server signature
	 * @return true if the data and signature match
	 */
	public static boolean verify(PublicKey publicKey, String signedData, String signature) {
		Signature sig;
		try {
			sig = Signature.getInstance(SIGNATURE_ALGORITHM);
			sig.initVerify(publicKey);
			sig.update(signedData.getBytes());
			if (!sig.verify(Base64.decode(signature))) {
				logger.error("Signature verification failed.");
				return false;
			}
			return true;
		} catch (NoSuchAlgorithmException | InvalidKeyException | SignatureException | Base64DecoderException e) {
			logger.error("", e);
		}
		return false;
	}
}
